This invention relates generally to signal processing devices that measure the phase difference between two coherent beams. More particularly, it relates to methods and apparatus for measuring and compensating for the effects of atmospheric turbulence on interferometric measurements.
Interferometers typically can measure the change in position of a movable measurement mirror with respect to a second stationary reference mirror. To perform this measurement, a light source generates two beams of light, one of which is reflected from the reference mirror and one of which is reflected from the measurement mirror. The light reflected from the two mirrors is then combined. If one mirror moves relative to the other mirror, the intensity of the combined beam periodically increases and decreases as the reflected light from the two paths alternately interferes constructively and destructively. This constructive and destructive interference is caused by the two beams moving in and out of phase. Each half wavelength of mirror movement results in a total optical path change of one wavelength and thus, in one complete cycle of intensity change. The number of cycle changes indicates the number of wavelengths that the mirror has moved. Therefore, by counting the number of times the intensity of the light cycles between darkest and lightest the change in position of the moving mirror can be estimated as an integral number of wavelengths.
However, a precise measurement of the position of the moving mirror requires the interferometer to determine the position of the mirror to a fraction of a wavelength. Theoretically, this can be achieved by measuring the phase difference between the two beams of light and converting this phase difference into a fraction of the wavelength. Prior art systems have measured the phase difference between two coherent beams by measuring the intensity of the combined beam. This intensity measurement indicates the degree of constructive or destructive interference between the two signals and the degree of interference is a measure of the phase angle between the two signals. Therefore, these systems measure the signal amplitude of the combined beam and relate the measured amplitude to the phase difference between signals,
Although theses prior art systems offer a theoretically sound way to measure the phase difference, in practice relating the signal amplitude to the phase difference between two light beams is a difficult feat. In particular, the moving mirror, the stationary mirror and the other optical components of the interferometer can create a noisy environment that effects the amplitude of the combined signal. Consequently, the measured amplitude may be imperfectly related to the actual amplitude of the combined signal and the calculated phase difference can be similarly inaccurate.
Alternatively, the interferometer can employ a heterodyne laser source. As used herein, the term "heterodyne laser" refers to a laser that produces at least two beams of light closely spaced in frequency (e.g., having a frequency difference or beat frequency in the approximate range of 1-20 MHz.), and being orthogonally polarized. A polarizing beam splitter directs one of the beams along the measurement path to the measurement mirror and one of the beams along the reference path to the reference mirror. A heterodyne sensor combines the light reflected from the two mirrors and detects the beat frequency. As long as the measurement mirror remains stationary, the beat frequency remains fixed. However, if the measurement mirror moves away, a predictable change in the beat frequency will result due to the Doppler effect. Changes in the number of frequency beats are directly related to the changes in position of the measurement reflector. Thus, the heterodyne interferometer can accurately measure a change in the measurement mirror's position relative to the reference mirror.
Over the last decade or so, interferometers have played an important role in integrated circuit fabrication. A principal tool used for mass production of integrated circuit chips is a lithographic stepper. During integrated circuit fabrication, a substrate is affixed to a movable stage. The lithographic stepper is the device which positions the stage underneath a high performance image projection system. Interferometers are used to sense the stage position and to control the stepper. Consequently, as manufacturers attempt to reduce the dimensions of the circuits being formed on substrates, and as registration tolerances are tightened, interferometers are required to provide more precise control for the stepper.
The precision with which interferometers can provide such position control has been significantly enhanced by technical advances in the design of various optical components, including lasers, and photosensors. However, the performance of interferometers is, nevertheless, limited by changes in optical path length due to atmospheric disturbances in the measurement and reference paths. Such atmospheric disturbances can be easily controlled with regard to the reference path by enclosing the portion of the interferometer which includes the reference mirror in a vacuum chamber. However, enclosing the entire system, including the movable measurement mirror, which is typically mounted on the stage assembly, in a vacuum chamber can be very expensive.
To solve this problem, prior art systems attempted to compensate for fluctuations in optical path length by using sensors which detect atmospheric fluctuations. However, such approaches have enjoyed only limited success because the sensors utilized can not be located directly in the measurement beam path, without affecting the quality of the principal measurement.
Accordingly, one object of the invention is to provide an improved interferometer having the capability of directly measuring changes in the optical path length resulting from atmospheric disturbances.
Another object of the present invention is to provide an improved interferometer that compensates for measurement errors resulting from temporally dependent atmospheric fluctuations along the optical beam path.
A further object of the invention is to provide an improved interferometer having the capability of directly measuring changes in optical path length resulting from atmospheric disturbances, concurrently with measuring a change in position of a movable measurement mirror with respect to a stationary reference mirror.
Yet another object of the present invention is to provide an improved system for measuring the phase difference between two coherent beams of light that decreases sensitivity to signal noise and that reduces the number of optical components required to make the measurement.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.